Photochromic or phototropic glasses, i.e., glasses which change color (darken) upon exposure to actinic radiation, customarily ultraviolet radiation, and return to their original state when removed from the radiation, were disclosed almost 20 years ago in United States Patent No. 3,208,860. That patent taught the action of silver halide crystallites, viz., AgCl, AgBr, and AgI, in glass bodies to impart the desired reversible transmission properties thereto. The preferred base glass compositions were stated to be included within the R.sub.2 O--Al.sub.2 O.sub.3 --B.sub.2 O.sub.3 --SiO.sub.2 field and consisted essentially, by weight on the oxide basis, of about 4-26% Al.sub.2 O.sub.3, 4-26% B.sub.2 O.sub.3, 40-76% SiO.sub.2, and R.sub.2 O is selected from the group of 2-8% Li.sub.2 O, 4-15% Na.sub.2 O, 6-20% K.sub.2 O, 8-25% Rb.sub.2 O, and 10-30% Cs.sub.2 O, the sum of those components comprising at least 85% of the total composition. With respect to developing photochromic behavior therein, at least one halide is present in a minimum effective amount of 0.2% Cl, 0.1% Br, and 0.08% I and silver is included in at least the aluminum indicated proportions of 0.2% where chloride is the effective halide, 0.05% where bromide is the effective halide, and 0.03% where iodide is the effective halide.
The most prevalent use of photochromic glass up to the present time has been in the fabrication of spectacle lenses, both as prescription lenses and as non-prescription sunglasses. Prescription lenses, marketed under the trademark PHOTOGRAY by Corning Glass Works, Corning, N.Y., constituted the first large volume commercial product. That glass has the following approximate analysis, tabulated in weight percent on the oxide basis:
______________________________________ SiO.sub.2 55.6 B.sub.2 O.sub.3 16.4 Al.sub.2 O.sub.3 8.9 Li.sub.2 O 2.65 Na.sub.2 O 1.85 K.sub.2 O 0.01 BaO 6.7 CaO 0.2 PbO 5.0 ZrO.sub.2 2.2 Ag 0.16 CuO 0.035 Cl 0.24 Br 0.145 F 0.19 ______________________________________
As can be appreciated, that comparison reflected the need for compromises to be drawn between a number of chemical and physical properties such as glass melting and forming capabilities, glass quality, chemical durability, and ophthalmic requirements, as well as photochromic behavior. Consequently, research has been continuous to devise glass compositions demonstrating improved photochromic properties, while retaining the basic chemical and physical characteristics demanded in a glass designed for the production of ophthalmic products.
One result of that research has been the development of prescription lenses marketed by Corning Glass Works under the trademark PHOTOGRAY EXTRA which darken to a lower luminous transmittance and fade more rapidly than the PHOTOGRAY brand lenses. The glass for those lenses falls within the composition ranges disclosed in U.S. Pat. No. 4,190,451. That patent describes glasses which, in 2 mm cross section, display the following properties:
(a) at a temperature of about 20.degree. C. will darken to a luminous transmittance below 40% in the presence of actinic radiation; will fade at least 30 percentage units of transmittance after five minutes' removal from the actinic radiation; and will fade to a luminous transmittance in excess of 80% in no more than two hours after being removed from the actinic radiation;
(b) at a temperature of about 40.degree. C. will darken to a luminous transmittance below 55% in the presence of actinic radiation; will fade at least 25 percentage units of transmittance after five minutes' removal from the actinic radiation; and will fade to a luminous transmittance in excess of 80% in no more than two hours after being removed from the actinic radiation; and
(c) at a temperature of about -18.degree. C. will not darken to a luminous transmittance below 15% in the presence of actinic radiation.
The glasses consist essentially, expressed in terms of weight percent on the oxide basis, of
______________________________________ Li.sub.2 O 0-2.5 Na.sub.2 O 0-9 K.sub.2 O 0-17 Cs.sub.2 O 0-6 Li.sub.2 O + Na.sub.2 O + K.sub.2 O + Cs.sub.2 O 8-20 B.sub.2 O.sub.3 14-23 Al.sub.2 O.sub.3 5-25 P.sub.2 O.sub.5 0-25 SiO.sub.2 20-65 CuO 0.004-0.02 Ag 0.15-0.3 Cl 0.1-0.25 Br 0.1-0.2 molar ratio alkali metal oxide:B.sub.2 O.sub.3 0.55-0.85 weight ratio Ag:(Cl + Br) 0.65-0.95 ______________________________________
Another particular advantage exhibited by those glasses is their relative temperature independence. Hence, in many compositions the photochromic properties vary widely with the ambient temperature. For example, at temperatures of about 40.degree. C. (104.degree. F.) some glasses will darken only a few percentage units; at temperatures of about -18.degree. C. (0.degree. F.) others darken to 5% or less. Such glasses are of little practical utility. The PHOTOGRAY EXTRA brand glasses combine relatively wide swings of darkening and fading with comparatively little temperature dependence.
PHOTOGRAY EXTRA brand lenses have the approximate composition, expressed in terms of weight percent on the oxide basis, of
______________________________________ SiO.sub.2 55.8 Al.sub.2 O.sub.3 6.48 B.sub.2 O.sub.3 18.0 Li.sub.2 O 1.88 Na.sub.2 O 4.04 K.sub.2 O 5.76 ZrO.sub.2 4.89 TiO.sub.2 2.17 CuO 0.011 Ag 0.24 Cl 0.20 Br 0.13 ______________________________________
PHOTOGRAY and PHOTOGRAY EXTRA lenses have been fabricated in like manner to the process followed in the formation of conventional, i.e., non-photochromic, opthalmic lenses. Thus, a glass blank is pressed and the blank thereafter ground and polished to a predetermined prescription. The glass blank is heat treated in a predefined schedule to develop photochromic properties therein.
U.S. Pat. No. 4,018,965 and 4,130,437 disclosed that non-prescription photochromic lenses, e.g., sunglass lenses, could be prepared without the above-described shaping of prescription lenses via grinding and polishing. Both patents described potentially photochromic glass compositions which are capable of being drawn into glass sheet; the expression potentially photochromic signifying glass which, as drawn into sheet, is virtually non-photochromic but which, after being subjected to an appropriate heat treatment, will evidence photochromic behavior.
The glass compositions disclosed in each patent manifest a viscosity at the liquidus of at least 10.sup.4 poises, customarily about 10.sup.4 -10.sup.6 poises, and display long term stability against devitrification when in contact with platinum metal and high temperature refractory ceramic materials at temperatures responding to glass viscosities in the interval of 10.sup.4 -10.sup.6 poises. As defined herein, long term stability against devitrification is deemed to reflect the growth of a layer of crystals no more than 10 microns in thickness at the glass-metal or refractory interface after a 30-day contact. This resistance to crystallization is demanded because the preferred sheet forming practice involves the use of a downdraw fusion pipe such as is described in U.S. Pat. Nos. 3,338,696 and 3,682,609 which may be fabricated from platinum or high temperature refractory ceramic material. Hence, that method, in like manner to other commercially-utilized sheet glass drawing methods, does not impart the very fast quenching action of conventional pressing processes for shaping glass articles. That factor inherently incurs an uncertainty with respect to the feasibility of preparing haze-free, highly-darkening, potentially photochromic glass sheet.
The glasses of each patent have the capacity for being chemically strengthened, when present as sheet in thickness between about 1.3-1.7 mm, to thereby comply with the standards for eyeglass lens safety mandated by the Federal Food and Drug Administration (FDA).
The preferred, potentially photochromic glass compositions of U.S. Pat. No. 4,130,437 can be simultaneously heat treated to effect photochromic properties therein and to sag the glass into molds to produce spectacle lens blanks of the desired curvature, such as is disclosed in U.S. Pat. No. 4,088,470. That capability, coupled with the faculty of being formed into sheet, has permitted the rapid and economical fabrication of sunglass lenses.
The operable ranges of glass compositions delineated in U.S. Pat. Nos. 4,018,965 and 4,130,437 are tabulated below in terms of weight percent on the oxide basis:
______________________________________ U.S. Pat. U.S. Pat. No. 4,018,965 No. 4,130,437 ______________________________________ SiO.sub.2 54-66 54-66 Al.sub.2 O.sub.3 7-15 7-15 B.sub.2 O.sub.3 10-25 10-25 Li.sub.2 O 0.5-4 0.5-4 Na.sub.2 O 3.5-15 3.5-15 K.sub.2 O 0-10 0-10 Li.sub.2 O + Na.sub.2 O + K.sub.2 O 6-16 6-16 PbO 0-3 0-1.25 Al 0.1-1 0.1-0.3 Cl 0.1-1 0.2-1 Br 0-3 0-0.3 CuO 0.008-0.16 0.002-0.02 F 0-2.5 0-2.5 ______________________________________
U.S. Pat. No. 4,168,339 is concerned with the production of photochromic glass in the form of microsheet, i.e., sheet glass having thicknesses between about 0.25-0.5 mm. The sheet drawing process employed is again that described in U.S. Pat. Nos. 3,338,696 and 3,682,609, supra, using glasses, expressed in terms of weight percent on the oxide basis, consisting essentially of:
______________________________________ SiO.sub.2 54-66 Al.sub.2 O.sub.3 7-16 B.sub.2 O.sub.3 10-30 Na.sub.2 O 3-15 Li.sub.2 O 0-4 K.sub.2 O 0-10 PbO 0.4-1.5 Br 0.2-0.5 Cl 0.5-1.2 F 0.2-0.5 CuO 0.008-0.03 Ag &gt;0.03-1 ______________________________________
U.S. application Ser. No. 252,139, now Pat. No. 4,358,542, filed Apr. 8, 1981 in the names of George B. Hares, David J. Kerko, and David L. Morse under the title PHOTOCHROMIC GLASS SUITABLE FOR MICROSHEET AND SIMULTANEOUS HEAT TREATMENT AND SHAPING, is also directed to the production of photochromic glass in sheet form utilizing the above-described overflow downdraw fusion pipe. Where desired, the glass sheet can be simultaneously heat treated to induce photochromic behavior therein and to sag the glass into molds to form eyeglass lens blanks of the proper curvature.
Operable glasses, expressed in terms of weight percent on the oxide basis, consist essentially of:
______________________________________ SiO.sub.2 58.5-60 Al.sub.2 O.sub.3 9-10 B.sub.2 O.sub.3 19-20.5 Li.sub.2 O 2-2.5 Na.sub.2 O 2-3 K.sub.2 O 6-7 PbO 0.1-0.25 Ag 0.1-0.15 Cl 0.3-0.5 Br 0.05-0.15 CuO 0.0065-0.01 ______________________________________
With respect to photochromic behavior:
The glasses of U.S. Pat. No. 4,018,965 are stated to exhibit at ambient temperatures, i.e., 20.degree.-25.degree. C., a clear luminous transmittance of at least 60%, a darkened luminous transmittance not exceeding 25%, and a fading rate such that the glass evidences a faded luminous transmittance after a five-minute fading period from the darkened state of at least 1.5 times that of the darkened transmittance;
The glasses of U.S. Pat. No. 4,130,437 are asserted to demonstrate at temperatures of 20.degree.-25.degree. C. a clear luminous transmittance of at least 60%, a darkened luminous transmittance below 30%, a fading rate such that the glass manifests a faded luminous transmittance after a five-minute interval from the darkened state of at least 1.75 times that of the darkened transmittance, and, after a one-hour fading period, the glass evidences a luminous transmittance greater than 80% of its clear luminous transmittance;
The glasses of U.S. Pat. No. 4,168,339 are observed to demonstrate a darkened luminous transmittance at room temperature below 50% and a fading rate such that, after a five-minute fading time, the transmittance will have increased by at least 20 percentage units and, after a one-hour fading interval, the glass will display a luminous transmittance in excess of 80%;
The glasses of Ser. No. 252,139 are noted to exhibit a clear luminous transmittance greater than 65%, a darkened luminous transmittance at 20.degree. C. below 25%, a fading rate at 20.degree. C. such that the glass demonstrates a faded luminous transmittance of at least twice that of the darkened transmittance after a five-minute fading period, a darkened transmittance at 40.degree. C. below 45%, and a fading rate at 40.degree. C. such that the glass displays a faded luminous transmittance of at least 1.75 times that of the darkened transmittance after a five-minute fading interval.
It will be appreciated that the initial clear luminous transmittance levels of the glasses described in each of the above disclosures are in the vicinity of 90%. Various tinting agents such as transition metal oxides, e.g., CoO, Cr.sub.2 O.sub.3, Fe.sub.2 O.sub.3, MnO.sub.2, NiO, and V.sub.2 O.sub.5, and/or rare earth oxides, e.g., Er.sub.2 O.sub.3, Ho.sub.2 O.sub.3, Nd.sub.2 O.sub.3, and Pr.sub.2 O.sub.3, are added to the base glass composition to reduce the initial clear luminous transmittance values to levels approaching 60%. Inasmuch as the amount of tinting agent employed is so small, the overall photochromic properties of the glass are not significantly affected by such additions.
Although the photochromic properties of the above-cited glasses constitute substantial improvements upon the first marketed PHOTOGRAY brand lenses, the desire for even greater darkening and more rapid fading is ever present. And, as has been emphasized above, the search for such improved photochromic behavior must also lead to a glass exhibiting chemical and physical characteristics inherent in conventional ophthalmic lenses such that it can be produced, marketed, and used by the wearer in customary fashion. Those characteristics include the need to be chemically strengthenable to comply with the FDA standards for eyeglass lens safety and, in the case of prescription lenses, to have the proper refractive index.